The power blackout that hit large swathes of the US and Canada in August was "preventable", the US energy secretary has said.
The blackout caused transport chaos
Spencer Abraham said the blackout was largely the fault of Ohio-based plant operator FirstEnergy.
His statement was timed with the release of a three-month US-Canadian investigation into the shutdown, which affected 50 million people.
A draft bill proposed by US Government aims to prevent a similar blackouts.
"One major conclusion of the interim report is that this blackout was largely preventable," Mr Abraham said in a statement.
"However, the report also tells us that once the problem grew to a certain magnitude, nothing could have been done to prevent it from cascading out of control."
The blackout was triggered when FirstEnergy's Eastlake plant unexpectedly shut down on 14 August, the report said.
It highlights a series of problems, including the failure of FirstEnergy's alarm system which should have alerted employees to the transmission fault.
The shutdown cut off a major supply route, that feeds into a massive electrical grid. It led to blackouts in New York, Detroit, Cleveland, Toronto and Ottawa.
The Ohio shutdown "instantly created major and unsustainable burdens on lines in adjacant areas," the 134-page report said.
Another contributing factor was the company's failure to trim trees that short-circuited three power lines in Ohio, investigators said.
"These failures helped create a problem of such magnitude as to be insurmountable," Mr Abraham said.
The report said FirstEnergy violated four specific voluntary standards set by the North American Electric Reliability Council (Nerc).
Another utility company, Midwest grid operator, was found to have violated two of the industry standards.
Communication was also a problem, the report said. Midwest lacked up-to-date information on what was happening with FirstEnergy's power lines, slowing attempts to stop the power failure from spreading.
The draft legislation put forward by Republicans in the US, would set tougher new reliability standards aimed at preventing a similar power shutdown.
The proposals are part of a wide-ranging energy bill expected to be voted on next week by the full House and Senate.